The present invention relates generally to hearing aids, and more particularly, to circuits that more compactly, economically, and effectively present a modified and amplified sound for the wearer of a hearing aid. In its most basic form, a hearing aid is a device which receives a sound signal and furnishes a modified sound signal to the wearer of the hearing aid, so that the wearer may hear better.
Hearing impaired persons differ considerably in the degree and pattern of their hearing loss. This is reflected in their audiograms.
An audiogram is a chart of hearing threshold level (HTL) versus frequency. The HTL is measured on a logarithmic (decibel) scale and compares an individual's threshold of detection of a tone to that of normal hearing persons.
Audiograms can differ in level from near normal (0 dB HTL) to profound loss (greater than 100 dB HTL). They can vary in pattern from a flat audiogram (approximately equal HTL over the relevant frequency range) to a sharply falling audiogram (HTL increasing at more than 30 dB per octave of frequency increase) or to a rising audiogram (HTL decreasing with frequency). Within the category of sharply falling audiograms, the frequency at which the audiogram begins to fall can vary greatly between individuals.
Patterns of hearing loss can also differ considerably in the relationship between the subjectively experienced loudness and the input sound pressure level. Not only are there wide differences in the lowest level that the ear can perceive (the HTL), there are also wide differences in the highest level that the ear can tolerate (the loudness discomfort level or LDL) and differences in the rate of growth of loudness between these two extremes. In some cases, a much higher than normal HTL is combined with a lower than normal LDL, giving a much reduced dynamic range of usable sound levels.
Because of the large differences in degree and pattern of hearing loss, there is also a wide range of hearing aid characteristics required to optimally assist hearing impaired persons. The frequency response of the hearing aid must be selected for the individual hearing loss and may require high order filtering with selection of frequency bandwidth as well as general shape within that bandwidth. The gain of the hearing aid must be selected in accordance with the degree of the hearing loss.
The limiting level of the hearing aid must be selected in accordance with the LDL. A selection of the type of output limiting must be made between peak clipping, or output Automatic Gain Control ("AGC"). Output AGC is an automatic gain control system whose action is related to the output level of the hearing aid. This relation is substantially independent of the setting of the user operated volume control.
If the ear has a much reduced dynamic range, an input AGC system may be needed. Input AGC is an automatic gain control system whose action is related to the input level to the hearing aid. The relation between the action of the input AGC and the input level is also substantially independent of the setting of the user-operated volume control. In the case of an input AGC, a selection must be made of the compression threshold, (the input level at which AGC action begins), and the compression ratio, (the ratio of the decibel change in input to the decibel change in output level).
It is desirable that a single hearing aid be provided with a range of adjustability in these various characteristics. This is important not only because it allows a single hearing aid model to be used for many persons with differing hearing losses, but also because it allows the hearing aid to be readjusted if the initial selection of hearing aid characteristics was incorrect or if the user's hearing loss changes with time.
Because hearing aids are normally worn on the head, they must be small and usually operate with a small single cell battery for extended periods of time. They must also provide good performance, be highly reliable, and be low in cost. The electronic circuits used to provide the various functions of a hearing aid must also have these same characteristics. Of course, such circuits will also be useful in applications other than hearing aids but having similar requirements.
Prior hearing aids have had limitations in meeting the needs described above. They have been limited in the degree of filtering provided to control the frequency response. When equipped with an input AGC system, they have not provided a well-defined compression threshold and a well defined compression ratio. Also, they have not provided the needed degree of adjustability in either their frequency response characteristics or in their output level versus input level and AGC characteristics.
Prior electronic circuits also have had limitations in meeting the needs described above. Circuits that are small in size and that can operate with a low supply voltage of about 1.3 volts and that draw little supply current have not provided good performance and have not provided the desired adjustment characteristics. Prior electronic circuits that could meet the performance needs described above have required higher supply voltage and current, have required many components, have been large in size, or have not been in a form in which they could be adjusted by operation of a single control.
The subsystems, or specific circuits, which help achieve the general objectives for hearing aids have their own specialized goals. Such specific goals are discussed in more detail below.